Data rates continue to increase in digital systems, communication systems, computer systems, and in other applications. In such applications, various devices may communicate by sending or receiving signals that are encoded with information in the form of signal levels (e.g., amplitude) in certain intervals of time. Proper decoding of periodic signals, for example, may involve measuring the signal level in the correct time interval, or period. As data rates increase, margins of error in the signal level timing tend to decrease, and the problem of accurately accounting for various imperfections in the data signal quality becomes more and more important.
In some communication systems, data can be formatted as a sequence of NRZ bits. Applications such as spread-spectrum communications, security, encryption and modems sometimes require the generation of bit sequences. In various applications, a specific bit sequence may serve as a test signal, for example. In some cases, a particular bit sequence may be repeated multiple times. One type of repeating bit sequence is a pseudo-random binary sequence (PRBS). Pseudorandom binary sequences are well known in the art and can be generated using linear feedback shift registers (LFSR). Sequences generated by an LFSR are actually “pseudo” random because the generated sequence is completely deterministic and repeats, and so is not truly random.
A digital oscilloscope (DO) is a tool that engineers can use to acquire and view signal waveforms from electronic circuitry. As signals get ever faster, it is beneficial to have DOs capable of digitizing these faster signals. The capability of a DO to digitize fast signals is generally a function of its effective bandwidth and effective sample rate. The sample rate is typically the number of samples points taken of a waveform acquired per unit time, and is equal to the reciprocal of the sample period, or the time between samples.
Various techniques have been developed for a DO to acquire waveforms. One such method is referred to as single shot mode. In single shot mode, a DO begins sampling a waveform event in real time when a trigger condition is satisfied. Such single shot acquisition is generally limited by the effective sampling rate of the analog-to-digital conversion, and the length of time over which an event may be sampled is limited by the size of the acquisition memory that receives the output from the converter.
Other methods for a DO to acquire waveforms may be referred to as random interleave sampling (RIS) or equivalent time (ET) sampling. In such modes of operation, a DO repeatedly samples a recurring event at different points (delays relative to a trigger event) in the event. A single composite representation of the event is then compiled from each of the samples. In order to reduce the probability of aliasing, the time between detection of the trigger condition and the sampling may be randomized between samples. An alternative method of equivalent time sampling is called coherent interleaved sampling, in which there is no trigger for each event; but rather a process of regular synchronized sampling at progressively advancing (or preceding) phase in the repeating pattern such that a composite acquisition can be formed of a similar nature to traditional equivalent time sampling techniques (see U.S. patent application Ser. No. 11/346,854, titled COHERENT INTERLEAVED SAMPLING, filed Feb. 3, 2006, currently pending, the contents thereof being incorporated herein by reference.